The present invention relates to a method for creating a patterned concrete surface. The method provides innumerable possibilities for shaping the surface structure of concrete in formwork and casting techniques. The method of the invention can be applied both when casting prefabricated units and in in-situ casting.
Previous use has been made of the surface retarding of concrete in concrete formwork and casting techniques, to create an exposed-aggregate concrete surface. Concrete surface retarding agents are existing substances, which retard the hardening of the concrete. They are used to create an exposed-aggregate concrete surface.
When casting prefabricated concrete units among other things, paper on which a surface retarding agent has been spread evenly is used on the bottom of the formwork, to create compact exposed-aggregate concrete units.
A surface retarding agent is also apparently used in certain applications, in such a way that the surface retarding agent is spread mechanically by hand through some kind of stencil onto the bottom of the formwork, thus creating some individual image.
Methods also exist, by means of which patterned exposed-aggregate concrete can be manufactured within predetermined limits, by using the surface retarding agent technique. Two such methods, which have something in common with the present invention, are disclosed in detail in the following. However, the publications referred to are clearly technically and economically inferior to the present invention.
The idea of one method is disclosed in U.S. Pat. No. 4,055,322. In this method, the surface retarding agent is spread on a water permeable membrane. The membrane is placed in the formwork, in such a way that the surface retarding agent is directly against the formwork surface, and the concrete is cast on top of the membrane. In order to function, the surface retarding agent must travel through the membrane to the surface of the concrete by means of diffusion, aided by water that has bled from the concrete.
The weakness of this method is that the need for water permeability limits the materials that can be used, on which the surface retarding agent is spread. In addition, the requirements of the fresh concrete also include a sufficient bleeding of water, which is difficult to control. This is particularly the case in present concrete technology, in which precisely the concrete grades that ensure a long-term durability feature a very small degree of water bleeding. On the other hand, a permeable membrane also weakens the final result, as the movement of the retarding agent inside the membrane also in the direction of the formwork is not taken into account in the patent and probably cannot be prevented. Thus, there is no precise boundary between the exposed and unexposed surfaces. Overall, it appears that the control of the behaviour of the surface retarding agent in the method according to the patent referred to is decisively poorer than in the method according to the patent now being applied for.
In practice, the formwork material immediately against the concrete on all smooth-cast concrete surfaces, made by the method according to U.S. Pat. No. 4,055,322, has been the water permeable membrane referred to above. The membrane may have an undesirable, and at least uncontrollable, effect on the quality of the smooth-cast surface.
The idea of the other method is disclosed in patent EP-0052237. In this method, a normal surface retarding agent membrane is used, with a surface treatment used to cover the part of the membrane on which an exposed-aggregate surface is not desired.
This method is uneconomical, in that a surface retarding agent membrane is required for the entire area to be patterned. In fact, the smooth-cast surface has not only the membrane, but also two layers of chemicals, i.e. the surface retarding agent and a varnish. In addition, the method referred to differs decisively form the present invention, in as much as only one surface retarding agent can be used at a time.
The present invention utilizes a surface retarding agent in an attempt to create a new concept, in which the surface retarding agent is applied either with a printing technique or an output technique.
The method now disclosed also provides, unlike its predecessors, an excellent opportunity of influencing the properties of the smooth-cast surface now created. As is generally known, the release agent spread on the surface of the formwork has its own effect on the quality of the smooth-cast surface created. Several different release agents have been developed to suit different formwork materials and grades of concrete. Release agents can be roughly divided into the following groups, for example:
pure mineral oils
chemically modified vegetable oils
mixtures of vegetable and mineral oils
emulsified mineral oils
emulsified vegetable oils.
In the method now disclosed, a suitable release agent can be spread on parts of the membrane, on which the surface retarding agent is not spread. This creates the desired quality of smooth-cast surface.
Another advantages of this embodiment is that, in prefabricated units, in which there will be no patterning at all, it is sufficient to spread the same release agent as that used on the membrane onto the surface of the smooth formwork. The methods previously referred to have not had this advantage, instead, in both methods, if it is desired that the surface of an entirely smooth-cast unit is identical with the smooth-cast surface of a partly exposed-aggregate unit, membranes must also be spread on these units. Here too the concrete manufacturer cannot influence the quality of the smooth-cast surface.
The method according to the invention is intended to achieve a solution with the following properties:
1. the method can be used to create a new kind of surface treatment for concrete in concrete casting technology, which allows the joints between units and the various graphic relief surfaces to be taken into account as part of the design
2. the method provides innumerable possibilities for the designer to shape the surface texture of the concrete with various graphic patterns
3. the method is as flexible as possible, and can be applied to different kinds of concrete casting
4. the point of departure of the method is the most designer-friendly production possible. The designer may send the finished design over a data network to the factory where it will be produced
5. a central objective, concerning the runoff of rainwater and the dirtiness of city air, is to anticipate the dirtying of the facade already during design
6. significant additional costs are avoided, compared to a conventional concrete surface.
The method is based on using concrete surface retarding agents, and, if desired release agents or other desired substances, either by printing techniques or output techniques, in formwork and casting technology. The desired surface pattern is transferred to the casting surface of the formwork, either by printing techniques or output techniques, in which a concrete surface retarding agent is used as the pigment. Modem printing and output techniques make it possible, if desired, the simultaneous or sequential addition of several different surface retarding agents or other substances, which have different effects on the concrete surface. This is because the basic principle of the invention includes not only the formation of a certain pattern, but also the surfacing of areas, to which a surface retarding agent is not added, with a release agent or other substance affecting the concrete surface and, in turn, the quality of the concrete piece created. The surface retarding agents may be of types that affect at different depths.
The casting surface of the formwork may be especially a membrane-like material, for example, coated paper or some other material, to which the pattern is transferred either by a printing or an output technique, in which a surface retarding agent acts as the printing agent or output agent. The technique may be serigraphy, flexography, a digital output technique, or any other printing or output technique. Once the cast concrete has hardened, the formwork is released and the concrete piece washed, to form a pattern on the parts of the exposed-aggregate surface, in which the surface retarding agent was in the formwork.
The method has several advantages over previously known methods of patterning concrete surfaces. Known methods of creating patterns in prefabricated concrete units have included various profiles attached to the bottom of the formwork, surface retarded spread with a brush through some stencil on the bottom of the formwork, sand-blasted patterns on the surface of the concrete unit, or the mechanical grooving of the concrete units. These previously known methods are largely manual processes and thus time-consuming and prevent less restricted patterns being created on the concrete surface. Compared to the state of the art of the publications referred to above, the method of the present invention is more diverse, and economically and ecologically more rational.
The new method according to the invention permits more highly automated production and innumerable possibilities for varying the patterns. It can be used to create a three-dimensional effect on the surface of a concrete piece being manufactured, as the invention makes it easy to create various of depth effects, as disclosed later with reference to the drawings and examples of embodiments.